As a member of the army commission, reporter of the budgets of the ministries of the interior and of agriculture he brought forward bills for the better treatment of the insane, for the establishment of a colonial ministry, for the taxation of alcoholic beverages, and for the reparation of judicial errors. He advocated complete freedom of the theatre and the press, the abolition of public executions, and denounced political corruption of all kinds. However, he was indirectly implicated in the Panama scandals through his father-in-law; as soon as he learned that he was benefiting by fraud, he made appropriate restitution.

However, Reinach is best known as the champion of Alfred Dreyfus. At the time of the original trial, he attempted to secure a public hearing of the case, and, in 1897, he allied himself with Auguste Scheurer-Kestner to demand its revision. He denounced in the Siècle the Henry forgery and Esterhazy's complicity. His articles in the Siècle aroused the fury of the anti-Dreyfus party, especially as Reinach was a Jew and was accused by some of taking up Dreyfus's defence on racial grounds.

He lost his seat in the Chamber of Deputies and, having refused to fight Henri Rochefort, an anti-Dreyfus journalist, eventually brought an action for libel against the latter. Finally, Dreyfus was pardoned, Reinach wrote a history of the case that was completed in 1905. Henri Dutrait-Croyon's Joseph Reinach, historien (Paris: A. Savaète, 1905), a violent detailed rebuttal of Reinach's history that "became a bible for that anti-Semitic section of French public opinion convinced of the culpability of Dreyfus."[1]

In 1906, Reinach was re-elected for Digne. In that year, he became a member of the commission of the national archives, and the following year a member of the council on prisons. Reinach was a prolific writer on political subjects. He published three volumes on Léon Gambetta in 1884 and also edited Gambetta's speeches.